Recent advances in liquid crystal polarization grating (“LCPG”) technology have enabled the use of passive LCPGs, singly and in combination, to manipulate light, particularly in display applications (See, for example, U.S. Pat. No. 8,537,310 to Escuti, et al., which is incorporated herein in its entirety by reference). In general, passive LCPGs possess a permanent, continuously varying periodic polarization pattern to diffract incident light according to its polarization.
More recently, LCPGs have been combined with switchable liquid crystal (“LC”) devices to provide low Size, Weight, and Power (“SWaP”) beam steering devices (See, for example, U.S. Pat. No. 8,982,313 to Escuti, et al., and Boulder Nonlinear Systems white paper, “Core Technologies,” September 2014, http://bnonlinear.com/wp-content/uploads/2014/09/Core-Technologies-White-Paper.pdf, accessed 30 Sep. 2015, which are incorporated herein in their entirety by reference). As an example, by incorporating fast electro-optic half-wave polarization retarders as a switch to control the handedness of polarization of the incident light, switchable beam steering devices with faster speed and lower SWaP compared to existing mechanical solutions, such as rotating Risley prisms, can be achieved.
As described, for example, in U.S. Pat. Nos. 8,537,310, 8,982,313 and “Core Technologies” whitepaper, passive LCPGs generally consist of a nematic LC film that is surface aligned and UV-cured to present a permanent, continuously varying periodic polarization pattern. The structure of such LCPGs provides an in-plane, uniaxial birefringence n that varies with position (i.e., n(x)=[sin(πx/Λ), cos(πx/Λ), 0], where Λ is the period of the grating). Such transmissive gratings efficiently (e.g., with greater than 99% efficiency) diffract circularly polarized light to either the first positive or negative order, based on the polarization handedness of the incident light.
As used herein, “zero-order” light propagates in a direction substantially parallel to that of the incident light, i.e., at a substantially similar angle of incidence when the light is incident on an optical system along an optical axis of the optical system, and is also referred to herein as “on-axis” light. For example, if the incident light is normally incident on the LCPG in a direction parallel to the optical axis, “zero-order” or “on-axis” light would also propagate substantially normally with respect to the first polarization grating. In contrast, “non-zero-order light,” such as “first-order” light and/or “second-order light,” propagates in a direction that is not parallel to the incident light nor the optical axis of the optical system. In particular, the second-order light propagates at greater angles than the first-order light relative to the angle of incidence. As such, first- and second-order light are collectively referred to herein as “off-axis” light.
LCPGs may be transparent, thin film, beam splitters that periodically alter the local polarization state and propagation direction of light traveling therethrough. Notably, during diffraction, the LCPG causes the polarization handedness of the incident light to flip to its orthogonal counterpart. Such characteristics are in contrast to conventional polarizers, which operate by permitting light of a first polarization state to travel therethrough, but absorbing light of an orthogonal, second polarization state.
A combination of two LCPGs may be aligned in parallel or in antiparallel configurations. Specifically, a “parallel” LCPG arrangement means the respective birefringence patterns of the two LCPGs have substantially similar orientations. In contrast, an “antiparallel” polarization grating arrangement means one LCPG has a birefringence pattern that is inverted or rotated by about 180 degrees relative to that of the other LCPG.
Non-mechanical beam steering can be achieved with an alternating stack of linear LCPGs and electro-optic half-wave retardance switches, some embodiments of which are described in the aforementioned U.S. Pat. No. 8,982,313. Non-mechanical beam steering devices (also known as beam scanners) provide numerous benefits over traditional gimbaled mechanical scanners due to their vastly reduced SWaP requirements and their ability to perform random access scanning. To achieve non-mechanical beam scanning with LCPGs, a nematic or ferroelectric liquid crystal modulator having an electronically controllable retardance is typically used as the retardance switch, as mentioned above. In this case, the retardance of the liquid crystal modulator is changed by applying a voltage to either produce a half-wave of retardance or nearly zero retardance through the cell. Since a half-wave retarder changes the handedness of circularly polarized light while a cell with no retardance does not affect the light's polarization, the incident light can be steered to a selected angle by controlling the handedness of circularly polarized light as it propagates through the LCPG stack. LCPGs have to date been demonstrated with apertures up to 50 mm.
It would be desirable to have alternative LCPG devices with further SWaP and performance advantages.